Nitration process for phenolic compounds

ABSTRACT

A phenolic compound, selected from phenol and m-cresol is nitrated to produce a high yield of paranitrated phenolic compound by a process comprising (1) providing a mixed acid solution having defined concentrations of HNO3, HNO2 and H2SO4, (2) adding the phenolic compound to said solution in a manner to avoid formation of an oil-phase therein while maintaining the temperature of the solution at from -20* to +25* C., the amount of phenolic compound added not exceeding 5 moles per liter of the acid solution and the concentration of HNO3 in said solution always being 1 mole per liter in excess of the phenolic compound added, and (3) recovering the paranitrated phenolic compound product from the reaction mixture.

O mted States Patent 1151 3,668,26 1 Harvey et al. 1 June 6, 1972 54]NITRATION PROCESS FOR PHENOLIC 2,301,912 11/1942 Jones "260/62 RCOMPOUNDS 3,326,983 6/1967 Vesely ..260/622 R 3,517,075 6/1970 Callister..260/622 R Inventors Ernest Albert Harvey, Beaver Falls, 3,510,5275/1970 Prosser ..260/622 R James Frederick Russ, North Brunswick, NJ.Primary Examiner-Bemard Helfin Assistant Examiner-W. D. Lone I f d, [73]Ass1gnee gislrl'lcan Cyanamld Company, Stam or Att0mey J0hn L sunwan[22] Filed: Dec. 17, 1969 [57] ABSTRACT [21] Appl. No.: 886,040 Aphenolic compound, selected from phenol and m-cresol is nitrated toproduce a high yield of paranitrated phenolic com- Relaed ApphcafionData pound by a process comprising (1) providing a mixed acid [62]Division of 509 629 Nov 24 9 Pat solution having defined concentrationsof l-lNO HNO and 'g i H 80 (2) adding the phenolic compound to saidsolution in a manner to avoid formation of an oil-phase therein whilemain- ..260 622 R taining the emperawre fmm m 3 C07c/79/24 C.,' theamount of phenolic compound added not exceeding 5 58] Fieid 509/629moles per liter of the acid solution and the concentration of HNO insaid solution always being 1 mole per liter in excess of the phenoliccompound added, and (3) recovering the [56] References Cltedparanitrated phenolic compound product from the reaction UNITED STATESPATENTS mixture 3,519,693 6/1970 Harvey et al. ..260/622 R 2 Claims, N0Drawings N ITRATION PROCESS FOR PHENOLIC COMPOUNDS This application is adivision of application Ser. No. 509,629 filed Nov. 24, 1965 now U.S.Pat. No. 3,5l9,693.

This invention relates to the nitration of phenolic compounds. Moreparticularly, it relates to an improved and economically advantageousprocess for the preparation of pnitrophenol and p-nitro-m-cresolinvolving the nitration of phenol and m-cresol, respectively.

The successful preparation of p-nitrophenol by the nitration of phenolwith little or no o-isomer formation is a problem of long standing inthe art. The method of attack for the most part has been through thepreparation of p-nitrosophenol followed by its oxidation .with nitricacid to p-nitrophenol. Nitrosation of phenol is known to take placepredominately in the para position and oxidation of para-nitrosophenolleads to the formation of p-nitrophenol. The major drawback to thismethod has been that while the yield of p-nitrosophenol is good (85-92percent), the subsequent oxidation to pnitrophenol gives only fair (60percent) yielddue to a large amount of tar formation.

Phenol and m-cresol are, of course, readily nitrated directly. Directnitration, however, leads to predominately the ortho isomer and theyields are poor due to excessive oxidation and tar formation. A study ofthe reaction by several authors has shown that the reaction is catalyzedby nitrous acid and that with nitric acid in only moderateconcentration, the presence of nitrous acid was, indeed, essential tothe reaction. Further, Bunton and coworkers, J. Chem. Soc., pps.2,646-2,648 1950), determined that the proportion of o-and pnitrophenolsin the product of the mononitration of phenol by nitric acid in stronglyacidified aqueous solution varied with the concentration of nitrousacid, being 73 parts o-nitrophenol to 27 parts p-nitrophenol whenessentially no nitrous acid was present and 9 parts o-nitrophenol to 91parts p-nitrophenol when the ratio of HNO -HNO was 2. While the para toortho ratios thus obtained were quite favorable, the yields ofmononitrated products were only in the neighborhood of 70-80 percent,thus, being not much better than the nitrosation followed by oxidationprocess. Furthermore, the mono-nitrated products were contaminated withlarge amounts of tar so that separation and recovery of the isomerproducts was extremely difficult.

It is seen, then, that the known processes for preparing nitrophenols,particularly p-nitrophenols, by the nitration of phenols have attainedonly limited success in terms of yield of p-nitrated products obtainablethereby. These processes have, therefore, not been sufficientlyattractive for commercial use. The provision of a nitration processwhich would be commercially useful would be animportant contribution tothe art. The present invention is concerned with such a process.

Accordingly, it is the primary object of this invention to provide a newand improved nitration process for the preparation of p-nitrophenolsfrom phenol and m-cresol. Other and further objects will become evidentfrom the following description of the invention.

It has now been found in accordance with the present invention that highyields (up to 100 percent) of mono-nitrated products comprised of up to92 percent of p-nitrated products can be obtained by nitrating phenol orm-cresol with nitric acid in the presence of nitrous acid and sulfuricacid under a specified combination of reaction conditions. Theseconditions are as follows:

l. Employing as the nitration medium an aqueous mixedacid solutioncontaining (a) from about 1 to about moles per liter, preferably about 3moles per liter, of nitric acid, (b) from about 0.4 moles to about 2moles per liter, preferably about 0.9 moles per liter, of nitrous acidand (c) from about 1.7 to about 6.4 moles per liter, preferably about2.3 moles per liter, of sulfuric acid; the mole ratio of nitrous tonitric acid in said solution being no greater than about 1.

. Adding the phenolic compound (reactant) to the aqueous mixed acidsolution in a manner (defined hereintion of an oil phase" in thereaction mixture during the course of the reaction, said oil phase beingcomprised of the phenolic compound reactant and/or the nitrated phenoliccompound product; the amount of phenolic compound added to said solutionbeing such that the moles of nitric acid present per liter of saidsolution always exceeds by at least l mole the moles of phenoliccompound added per liter of said solution; and, the total amount ofphenolic compound added being no more than 5 moles thereof per liter ofsaid acid solution.

3. Maintaining the nitration reaction mixture formed in (2) (above) at atemperature of from about 20 C. to about +25 C., and under a positivepressure, i.e., from 0 to about 100 p.s.i.g.

That the above combination of reaction conditions would provide such ahighly successful process for the para-nitration of the phenoliccompound could not have been predicted on the basis of the prior art.Thus, as mentioned hereinabove,

Bunton et al. (supra), had determined that the obtainment of a highratio of para to ortho isomer required a high ratio of nitrous to nitricacid in the nitration reaction. They further specified only a smallexcess of nitric acid over phenol. Contrary to that work, however,applicants have found that an excess of nitrous over nitric acid is notonly not necessary for obtainment of a high ,para to ortho isomer ratio,but that an excess of nitric over nitrous acid is actually advantageousfor reasons indicated hereinafter. In addition, and more importantly,applicants found' that a considerable excess of nitric acid over thephenol in reaction is necessary in order to limit tar formation in thereaction, a problem which Bunton et al. failed to solve. Furthermore,applicants found that the presence of an oil phase (comprised of eitherphenolic reactant or the nitrated phenolic product or both) in thereaction mass increases the formation of the ortho isomer product. Toavoid this, as previously stated, applicants conduct the nitrationreaction in a manner whereby the presence of an oil phase is eliminated.This is accomplished in one of the following ways.

1. Aqueous or liquid phenolic compound is added to the nitration medium,slowly or portion-wise, at a rate such that it reacts substantially asfast as it is added and its solubility limit in the acid is notsubstantially exceeded; the addition being made while maintaining thereaction mass at a sufficiently low temperature, i.e., from 20 to +10C., so that the reaction products precipitate as solids.

2. The phenol is added to the reaction medium, slowly or portion-wise,at such a rate that it reacts substantially as fast as it is added andthe solubility limit thereof in the acid is not exceeded; the totalquantity of phenol added being such that at the reaction temperature,i.e., 20, to +25 C., the solubilityof the products formed in the acid isalso not exceeded.

3. Solid phenolic compound is added at a sufficiently low temperature,i.e., from 20, to 0 C., so that it remains essentially solid and thereaction products also precipitate as solids.

The elimination of an oil phase in the nitration reaction in one of theaforesaid ways is an essential aspect of this invention which has notbeen suggested in the prior art.

It will be appreciated in connection with (l), (2) and (3) that whilethe temperatures designated therein overlap, the prevention of anysignificant oil phase formation is accomplished at the recitedtemperatures in each instance by controlling the rate of addition of thephenolic compound to the nitration medium. Thus, a slower addition rateis used at the lower portion of the temperature range in each case. Ingeneral, the time of addition can vary with successful avoidance of anysignificant amount of oil phase formation, over a period of from about10 minutes to 3 hours, depending on the reactant used (phenol orm-cresol) and the temperature of the reaction. Thus, in (l) thepreferred temperature below) which substantially completely avoids theformafor phenol as reactant is 5 C., and for m-cresol it is 0 C., the

preferred addition time in each case being about 3 hours (see Examples 1and 14). Addition times longer than 3 hours may of course be used,however they afford no particular advantage and are therefore notpreferred herein from a practi- Referring now to Table I, it is seenthat if any one of the reaction conditions specified in accordance withthe present invention is not met a decrease in yield of para isomerand/or an increase in tar formation occurs.

cal standpoint. 5 In the first place, it will be observed that an excessof nitric In procedure (2), on the other hand, where both the phenoloverphenol is needed to limit tar formation. Thus, it is seen ic reactantand the nitrated products are to be maintained in that in xample 1,where the preferred nitric acid/phenol solution, the total amount ofphenolic compound which can l ratio was Used. Product accountability W887 be added to the reaction is much lower than in (1), since the percentwith 91 percent thereof being para-nitrophenol and solubility limit ofthe nitrated product in the acid solution is only 6 percentortho-nitrophenol, he remainder. y 3 P quite low. Accordingly, highertemperatures (i.e., 10 to 25 Cent. belng tan C.) within the specifiedrange are preferred in order to permit Similarly, in Example 2, wherethe nitric acid/phenol ratio addition of the maximum amount of thephenolic compound was 4.05, only 0.3 percent tar was fomied. In acomparative without exceeding the solubility limit of the products. 1 5example, i.e., Example 3, where this ratio was lowered to 1.13, Althoughthis procedure provides equally high yields of paratar formation wasconsiderable, i.e., 44 percent. The same is nitrated products on thebasis of phenolic compound charged, true. th gh to a lesser g in Example9 and 2 it is less advantageous than (1) in terms of productivity" Wherethe 3/P ratio was at below the Prescribed because the amount of thephenolic compound charged is mmlmum necessarily much lower- With regardto the concentration of HNO it is seen that In Procedure Where thePhenolic Compound is added to where the other conditions were within thelimits specified for the reac i n m i m 21 a Solid. and the pr tObtained a the invention, good to excellent results were obtained bothin Solid, the temperature must, Of Course. be sufi'lciently low thatyield and low tar formation, even though the ratio of nitrous to thephenolic compound will remain solid prior to being taken nitric acid waslow. See Examples 1, 4 and 6. This low ratio, p i the reaction. notabove C 25 which is contrary to the higher ratio (2:1 called for by Bun-The criticality and inter-relationship of the combination of {on et 1 ia di i advantage f h present i the reaction conditions constituting thepresent invention are i b u it av id th use f hi h pressure a d r dillustrated by Examples i-13, the pertinent data with respect racticalproblems with respect to the handling and recovery to which aresummarized in Table I. ofnitrous acid.

In order to illustrate the preferred procedure for carrying With respectto the concentration of H 80 it will be seen out the invention, Example1 is described below in detail. Exthat where the conditions specifiedfor the invention are met, amples 2-13 were carried out in a similarfashion to Example i.e., the employment of a molar concentration thereofin the 1, except for the variation in conditions used as shown in acidsolution offrom 1.7M to 6.4M (the other conditions also Table I. beingwithin the limits specified), excellent to fair results are TABLE IMolarity of- Percent yield of- Ex. 'lump HNOz I'INO Time 01 NitrosoAccount- No. H 804 IINO IINO; HNO; Phenol phenol addition phenol DNP 0N1PNP ability Remarks (1) 5 2. 0. 89 2. 99 0. 30 1. 0 6 91 97 Best yield.(2). 20 1. 98 0. 2. 11 0. 21 0. 52 18. 7 80.0 99. 7 Good yield. (3)...20 1. 98 0. 45 0. 5i) 0. 77 0. 52 21. 8 5G. 0 Bad yield. (4). 1() 2. 350. 89 3. 00 0. 30 1. 00 3. 00 90 min. Trace Trace 8. 1 85. 5 94 Goodyield. (6) -10 1. 0. 89 0. 89 1. 0O 0. 41 2. 17 90 min. 56. 6. G 63. 2Bad yield. (6) 0 1. 0. 80 1. 80 0. 44 0. 5 7. 0 83. 0 90 Good yield.(7). 0 1. 70 0. 40 1. 00 0. 40 0. 5 5. 0 70. 0 Fair yield. (8).. 20 G.40 0. 45 7. 18 0. 06 1. 66 25. 1 71. 0 100. 1 D0. (9).. 10 6. 40 0. 892. 14 0. 42 1. 66 27. 0 58. 9 Bad yield. (10). 0 1. 70 0. 40 1. 4 0. 29O. 50 11. 0 74. 5 86 Fair yield. (11). 0 1. 7O 0. 8O 1. 4 0. 57 0. 50 7.5 81. O 89 Better yield. (12)-.. 0 2. 10 0. 4 1. 0 0. 40 0. 50 12. 3 71.2 83. 5 Fair yield. (13)-.. 0 2. 10 O. 4 1. 4 0. 29 0. 50 12. 1 77. 389. 4 Better yield.

EXAMPLE 1 obtained. In Example 5, however, where the H 50 concentrationused, 1.55M, was below that prescribed, i.e., 1.7M, the NITRATION OFPHENOL result obtained was poor, tar formation being 36.8 percent.

To a 65-gallon stirred and baffled kettle, 249 lbs. of ice were chargedand the kettle sealed. The following were then added with cooling tomaintain the temperature at -5i2 C.:

1. 59 lbs. of 80.5% I-INO (0.753 lb. moles) 2. 84 lbs. of 98.0% H SO,(0.840 lb. moles) 3. 29.2 lbs. of N 0, (0.3 18 lb. moles) To the aboveacid solution (2.35M H S0 0.89M HNO and 2.99M HNO there was added 46.6lbs. of phenol solution., (0.397 lb. moles) over 3 hours maintaining thetemperature at 5 1- 2 C., the product being precipitated as acrystalline solid. The reaction was heated to 25 C., and cooled to 0 C.,to enlarge the crystal size and convert any remaining traces ofnitrosophenol to nitrophenol. The whole was then filtered and washedwith cold water, employing suction to remove the water. The wet weightof product was 67.5 pounds. Analysis showed 73.4 percent nitrophenolwith a para to ortho isomer ratio of 16.7 to l, or 46.9 pounds (85percent yield) of para-nitrophenol. An additional 5-7 percent yield ofpara-nitrophenol was shown to be recoverable from the acid filtrate andwash water by extraction. Overall yield of paranitrophenol was -92percent.

With respect to reaction temperature, it is seen that good to excellentresults are obtainable over the range of 1 0 to +20 C., (Examples 1, 2,4, 6, l1 and 13). It is noted that in the reaction at 20 C. at which anoil-phase has more tendency to form than in the reaction at lowertemperatures, good results, are still obtained by increasing the ratioof nitric acid to phenol, as shown in Examples 2 and 8 ofTable I.

With regard to pressure, it is pointed out that the reaction must beconducted under a positive pressure (i.e., 0 to p.s.i.g.) in order tomaintain the required concentration of HNO therein. This is accomplishedby conducting the reaction in a closed reactor capable of withstandingthe nitrous acid pressure which thus accumulates in the reaction.

The following are further examples illustrative of the inventionincluding several modifications in process procedure which may be usedtherein.

EXAMPLE l4 NITRATION OF M-CRESOL To a round-bottom, three-neck flaskequipped with a stirrer, dropping funnel, thermometer and dry icecondenser, the following were charged:

1. 500 g.'of ice 2. 116 ml. (209 g.) of96.2% H SO [2.05 moles] 3 162.5ml. (231 g.) of 70.4% HNO [2.58 moles] The above solution was dilutedwith water to 850 ml. and 49.4 ml. (73.5g. or 0.8 mole) otN O, added.

To the above acid solution (2.05 M H SO 3.38 M HNO;, and 0.8 M HNO therewas added 120 ml. (123 g. or 1.1 moles) of 96-8 percent m-cresoldropwise over 3 hours while maintaining the temperature at i 2 C. Theprecipitated solids were filtered off, washed with cold water and thewater removed by suction. The product, dried overnight at roomtemperature, weighed 181. g. and analyzed 92 percent total nitro cresolsand 7 percent water, or l66.5 g. of nitro cresols (essentiallyquantitative conversion). Further analysis showed that the productcontained 87.4 percent 4-nitro--m-cresol, 5.2 percent 6-nitro-m-cresoland 4.7 percent 2-nitro-m-cresol or a para to ortho isomer ratio of 8.8.

EXAMPLE 15 NITRATION OF PHENOL To a l-liter graduated cylinder, chargethe following: l.500g.ofice 2. 189 g. (105 ml.) of96.2% H SO [1.9 moles]3. 80.5 g. (56.6 ml.) of70.4% HNO [0.9 mole] The above solution wasdiluted to 947 ml. with water at 0 C. and the whole charged to a2-liter, round-bottom, threenecked flask equipped with a stirrer,droppingfunnel, thermometer and dry ice condenser. 37.0 ml. of cold N 0(0.6 mole) were then added to the solution over minutes maintaining thetemperature of reaction at 0 C.

To the above acid solution 1.9M H 80 1.5M HNO and 0.6M HNO there wasadded 47.0 g. (0.5 mole) of phenol dissolved in 16.5 g. of water over3/4 hour while maintaining the 0 C. temperature. The mixture was thenstirred an addition 45 minutes and extracted with 240 ml. of iso-butylacetate. 1t was extracted twice more with 120 ml. portions of acetate.The extracts were then combined and washed four times with 150 ml.portions of 25 percent brine solution. Analysis of the acetate solutionshowed a 85.2 percent yield of pnitrophenol and a 9.0 percent yield ofo-nitrophenol for a para to ortho isomer ratio of 9.5

EXAMPLE 16 NlTRATlON OF PHENOL in this example the required proportionof nitrous acid was provided by supplying sodium nitrate to the mixtureof nitric and sulfuric acids rather than nitrogen tetroxide. Salts ofthe other alkali metals, i.e., lithium and potassium, may also be used.

To a 65-gallon stirred and baffled kettle, charge 266 pounds of ice andseal the kettle. The following were then added with cooling to maintainthe temperature at 0 i 2 C.

1. 64 lb. of 80.5% l-lNO (0.82 lb. mole) 2. 111 1b. of98% H 50. 1. 11lb. moles) 3. 23.5 lb. of NaNO (0.34 lb. mole) in 35 lb. ofwater To theabove acid solution (4.1N H SO 2.1N HNO 0.8N HNO and 0.8N NaHSO therewas added 53 pounds of 92 percent phenol in water (0.52 lb. mole) over2% hours maintaining the temperature at 0: 2 C. during which time theproduct precipitated as a crystalline solid. The reaction was heated to30 C. and cooled to 0 C., to enlarge the crystal size and eliminatetraces of nitrosophenol. The whole was then filtered, washed with cold HO employing suction to remove the water. The wet weight of product was73 pounds. Analysis showed a 85.1 percent yield of nitrophenols with apara to ortho isomer ratio of to 1, or 56.5 pounds (78 percent yield) ofpara-nitrophenol. An additional 5-7 percent yield of para-nitrophenolwas recovered from the acid filtrate and wash water by extraction.Overall yield of p-nitrophenol was 83-85 percent.

EXAMPLE l7 NITRATION OF PHENOL To a 2-liter, round-bottom, three-neckflask equipped with a stirrer, dropping funnel, thermometer and a dryice condenser, the following were added while maintaining thetemperature below 0 C.:

1. 750 g. of ice 2. 180 g. of 96.4% H 80. 1.75 moles) 3. 138 g. of N 01.50 moles) To the above acid solution (3.9N H SO 2.2N HNO and 0.6N HNOthere was added 120 ml. of 74 percent phenol in water (1.0 mole),dropwise, over 1% hours maintaining the temperature at 0 2 C. When 25ml. of the phenol solution had been added, an additional 3 ml. 0198 H 50(0.05 mole) and 30 ml. of 70% HNO (0.47 mole) were added dropwise tomaintain the effective nitric acid concentration At the end of thephenol addition, the reaction was heated to 35 C., and cooled to 0 C.The solid product was then filtered off and washed with cold water, thewater being removed with suction. The wet weight of product was 130.3 g.Analysis showed an 88.2 percent yield of nitrophenols with a para toortho isomer ratio of 9.2 or 104 g. (75 percent yield) of pnitrophenoland 11.2 g. of o-nitrophenol. An additional 12.7 g. (9 percent yield) ofp-nitrophenol and 1.9 g. of onitrophenol were recovered from the acidfiltrate and wash water by extraction. The overall yield was 84 percentof pnitrophenol.

EXAMPLE l8 NlTRATlON OF PHENOL To a 65-gallon stirred and baffledkettle, 338 pounds of ice was charged and the kettle sealed. Thefollowing were then added with cooling to maintain the temperature at 01 2 C.:

l. 50 lb. of 80.5% HNO (0.64 mole) 2. lb. of98% H 50 (0.80 lb. mole) 3.32.5 lb. ofN O (0.35 lb. mole) At the end of the addition of the N 0 thepressure on the kettle was 12-15 p.s.i.g.

To the above acid solution (4.1N H SO 2.1N HNO and 0.8N HNO there wasadded 53 pounds of 92 percent phenol in water (0.52 lb. mole) over 4hours, maintaining the temperature at 0 i 2 C. The reaction was thenheated to 30 C., and cooled to 0 C., to enlarge the crystal size of theprecipitated product. The product was then filtered off and washed withcold water, using suction to remove the water. The wet weight of productwas 74.7 pounds. Analysis showed it to contain 82.9 percent nitrophenolswith a para to ortho isomer ratio of 7.9 or 55. 1 pounds (77 percentyield) of pnitrophenol and 6.9 pounds of o-nitrophenol. An additional6-8 percent yield of p-nitrophenol was recovered from the acid filtrateand was water by extraction. The overall yield was 83-85 percentp-nitrophenol.

EXAMPLE l9 NTTRATION OF PHENOL To 500 pounds of acid solution (4.1N H 802.2N HNO 0.8N HNO saturated with p-nitrophenol at 0 C., in a sealed65-gallon stirred and baffled reactor there was added 51 pounds of 92percent phenol in water (0.5 lb. mole) over 3 hours while maintainingthe temperature at 0 1 2 C. The reaction was heated to 30 C., and cooledto 0 C., to enlarge the crystal size of the precipitated product. Theproduct was then filtered off and washed with cold water employingsuction to remove the water. The wet weight of product was 81 poundswhich analyzed 80 percent nitrophenols with a para to ortho isomer ratioof 7.6 or 57.0 pounds (83 percent yield) of p-nitrophenol and 7.5 poundsof o-nitrophenol.

Having now fully described the invention what is claimed as new andpatentable is:

I, A process for nitruting a phenolic compound selected from the groupconsisting of phenol and m-cresol to produce a predominantlypara-nitrated phenolic compound which comprises the steps of: (1)providing an aqueous mixed acid solution containing from about 1 toabout 10 moles per liter of nitric acid, from about 0.4 to about 2 molesper liter of nitrous acid and from about 1.7 to about 6.4 moles perliter of sulfuric acid; the mole ratio of nitrous to nitric acid in saidsolution being no greater than about 1; (2) adding said phenoliccompound in solid form to said mixed acid solution while maintztiningthe reaction mass at a temperature of from about to about 0 C. and undera pressure of from O to about 100 p.s.i.g., whereby formation of an oilphase in the reaction mass is substantially completely avoided and thenitrated phenolic compound product is precipitated from said solution insolid form, the amount of said phenolic compound added being such thatthe moles of nitric acid present per liter of said solution alwaysexceeds by at least 1 mole the moles of phenolic compound added perliter of said solution; the total amount of said phenolic compound addednot exceeding 5 moles per liter of said solution; and, (3) separatingthe nitrated phenolic compound product from the reaction mass byfiltration.

2. The process of claim 1, wherein the mixed acid solution provided instep 1) contains about 3 moles per liter of nitric acid, about 0.9 molesper liter of nitrous acid and about 2.3 moles per liter of sulfuric acidand wherein the amount of phenolic compound added in step (2) is about 1mole per liter of said mixed acid solution.

2. The process of claim 1, wherein the mixed acid solution provided instep (1) contains about 3 moles per liter of nitric acid, about 0.9moles per liter of nitrous acid and about 2.3 moles per liter ofsulfuric acid and wherein the amount of phenolic compound added in step(2) is about 1 mole per liter of said mixed acid solution.